Backup power system

ABSTRACT

A power backup processor and a method of operating the power backup processor suitable for a single-phase AC power supply and with a higher efficiency, at least when the critical load is fed with the single-phase AC power supply, while having a high availability and quality of voltage supply. This can avoid the drawbacks related to high DC voltages.

The present invention relates to a power backup processor unitcomprising a first AC input line comprising a first inductance andprovided to be connected to an AC power supply, a first AC output linecomprising a second inductance and provided to be connected to acritical load, a second AC output line provided to be connected to thecritical load, a neutral line provided to be connected to a neutralport, first and second DC voltage busses provided to be connected to aDC power supply, a capacitor interposed between the first and second DCvoltage busses, a first half-bridge circuit comprising first and secondswitches, wherein the first switch is connected between the first ACinput line and the first DC voltage bus and the second switch isconnected between the first AC input line and the second DC voltage bus,a second half-bridge circuit comprising third and fourth switches,wherein the third switch is connected between the first AC output lineand the first DC voltage bus and the fourth switch is connected betweenthe first AC output line and the second DC voltage bus, a thirdhalf-bridge circuit comprising fifth and sixth switches, wherein thefifth switch is connected between the neutral line and the first DCvoltage bus and the sixth switch is connected between the neutral lineand the second DC voltage bus. The processor further comprises a seriesof controllers provided to command the switches, wherein said series ofcontrollers is provided to command at least the first to fourth switchesusing pulse width modulation. The present invention also relates to amethod of operating said power backup processor.

Power backup processors are typically used in the telecommunicationindustry to ensure continuous delivery of AC energy to criticalapplications such as a central office of a telecommunication operator.

A configuration typically known from the state of the art and comprisinga series of converters is illustrated in FIG. 1. In this configuration,a first converter 101, which is an AC/DC converter, has a first AC inputand a first DC output. The first AC input comprises a first AC inputline 119 and a neutral line 120. Said first AC input line 119 isprovided to be connected to a single-phase AC power supply, whereas saidneutral line 120 is provided to be connected to a neutral port. A secondconverter 102, which is a DC/AC converter, has a first DC input and afirst AC output, which comprises a first AC output line 121 and a secondneutral line 122. Said first AC output line 121 is provided to beconnected to a critical load 108, whereas said second neutral line 122is provided to be connected to a neutral. A third converter 103, whichis a DC/DC converter, has a second DC input and a second DC output, saidsecond DC input being provided to be connected to a DC power supply. Afourth converter 104, which is another DC/DC converter, has a third DCinput and a third DC output. The first DC output is connected to thethird DC input, the third DC output to the second DC input and thesecond DC output to the first DC input.

The second converter 102 connected to the third converter 103 formtogether an inverter 100 b, and the first converter 101 connected to thefourth converter 104 form together a rectifier 100 a. A battery 106 istypically used as a DC power supply and therefore connected between theoutput from the rectifier 100 a and the input from the inverter 100 b. Aswitching unit 105 comprising switches 105 a and 105 b and amicro-controller 105 c to control the switches is foreseen to allow toswitch between a connection using the power processor formed by therectifier 100 a and inverter 100 b and a connection through the cable107. The critical load 108 is here illustrated by one resistance. Inpractise, the critical load may be formed by a plurality of unitsconnected in parallel to the processor.

A problem with this known configuration as illustrated in FIG. 1 is thatthe efficiency is relatively low. From the single-phase AC power supplyto the critical load, the current needs to pass through four convertersand one switch. The efficiency obtained through such a circuit istypically around 80%. When the critical load is fed with the DC powersupply, the current needs to pass through two converters and one switchyielding an efficiency in the order of 90%.

U.S. Pat. No. 4,709,318 discloses an alternative configuration whichcomprises just three converters, these three converters being a firstconverter, which is an AC/DC converter with a first AC input and a firstDC output, a second converter, which is a DC/AC converter with a firstDC input and a first AC output, and a third converter, which is a DC/DCconverter provided with a second DC input and a second DC output. Inthis alternative configuration of the state of the art, the first ACinput is provided to be connected to an AC power source, the first ACoutput is provided to be connected to a critical load, the second DCinput is provided to be connected to a DC power source and both thefirst and second DC outputs are connected to two DC voltage busses inturn connected to the first DC input. With this configuration, thecurrent only needs to pass through two converters between the AC powersupply and the critical load. However, in this disclosure the first andsecond converters are such that the voltage between the DC voltagebusses is comparatively high, namely 400 V DC to achieve a 120 V RMS ACoutput voltage. Such high voltage requires dimensioning the elements ofthe converters accordingly, increasing the cost while reducing bothefficiency and reliability.

United States Patent Application Publication US 2005/0162137 A1discloses a power backup processor comprising the features of thepreambles of the independent claims. In particular, this power backupprocessor comprises three AC phase input lines, each comprising aninductance and provided to be connected to a three-phase AC powersupply, three AC phase output lines, each comprising another inductanceand provided to be connected to a critical load, a first neutral line,first and second DC voltage busses provided to be connected to a DCpower supply, a capacitor interposed between the first and second DCvoltage busses, and a half-bridge circuit for each one of the three ACphase input lines, three AC phase output lines and first neutral line.Each half-bridge circuit comprises a switch connected between thecorresponding AC phase input, AC phase output or neutral line and thefirst DC voltage bus and another switch connected between the samecorresponding AC phase input, AC phase output or neutral line and thesecond DC voltage bus. This configuration also comprises a series ofcontrollers provided to command all switches using pulse widthmodulation. In operation, the half-bridge circuits corresponding to thethree AC phase input lines are commanded using pulse width modulation soas to generate, from the three-phase AC input, a nearly continuouspositive voltage at the first DC voltage bus and a nearly continuousnegative voltage at the second DC voltage bus. The half-bridge circuitcorresponding to the first neutral line is commanded merely tocompensate minor voltage fluctuations at the first and second DC voltagebusses, both switches of this half-bridge being closed only for a verysmall percentage of the time and in very short pulses. The first neutralline in this configuration therefore operates merely as a “0” voltsaxis. The normal object of this device is the zero sequence generationto reduce harmonic voltages on a three-phase output. These configurationand control method, apart from being suitable only for three-phase ACpower supplies, furthermore also require a comparatively high DC voltagebetween the first and second DC voltage busses, namely up to 862 V DCfor an output line-to-neutral voltage of 277 V.

It is a primary object of the present invention to provide a powerbackup processor and a method of operating said power backup processorsuitable for a single-phase AC power supply and with a higherefficiency, at least when the critical load is fed with the single-phaseAC power supply, while having a high availability and quality of voltagesupply, and avoiding the drawbacks related to high DC voltages.

To this end, the power backup processor unit further comprises a fourthhalf-bridge circuit comprising seventh and eighth switches, wherein theseventh switch is connected between the second AC output line and thefirst DC voltage bus and the eighth switch is connected between thesecond AC output line and the second DC voltage bus, the series ofcontrollers being provided to open the fifth switch and close the sixthswitch for a period t₁ of at least 50%, and preferably at least 95%, ofa first half of a cycle of V_(L,in) and close the fifth switch and openthe sixth switch for a period t₂ of at least 50%, and preferably atleast 95%, of a second half of said cycle of V_(L,in). Preferably, saidseries of controllers is also provided to open the seventh switch andclose the eighth switch for a period t₁′ of at least 50%, and preferablyat least 95%, of a first half of a cycle of V_(L,out) and close thethird switch and open the fourth switch for a period t₂′ of at least50%, and preferably at least 95% of a second half of said cycle ofV_(L,out). This arrangement generates a voltage V_(N,out) at the secondAC output line that runs countercyclically to V_(L,out). As a result,the voltage V_(Load) between the two connections to the load has anamplitude that is the sum of those of V_(L,out) and V_(N,out). Anefficiency in the order of 90% can be obtained with the AC power supply.In addition, this circuitry has fewer components and is hence simplifiedwhen comparing it with prior art circuitries. A further consequence isthat it allows having simultaneously a sinusoidal input current (withoutharmonics) while having a non linear critical load at the outputsupplied by a sinusoidal voltage with a non sinusoidal current (withcurrent harmonics). This insures proper harmonic filtering of the ACinput voltage and simultaneously the AC output current. It furtherallows proper functioning when a small phase shift between input (ACpower supply) and output (at the critical load) is present as well as incase small voltage differences between input and output occur.

Alternatively, the neutral line is provided to be directly connected tothe second AC output line, and the power backup processor furthercomprises a third inductance connected between the neutral line and thethird half-bridge circuit, the series of controllers being provided toopen the fifth switch and close the sixth switch for a period t₁ of atleast 50%, and preferably at least 95%, of a first half of a cycle ofV_(L,in) and close the fifth switch and open the sixth switch for aperiod t₂ of at least 50%, and preferably at least 95%, of a second halfof said cycle of V_(L,in). This second embodiment also generates aV_(N,out) that is countercyclical to V_(L,out). While in this simplerarrangement it is not possible to change the frequency and phasedisplacement of V_(L,out) with respect to V_(L,in), the reduction in thenumber of components further increases the reliability, as well as theefficiency, which can reach around 95%.

Advantageously, the fifth and sixth switches and/or the seventh andeight switches are pulse-width moderated during a period t₃,respectively t₃′, between t₁ and t₂, respectively t₁′ and t₂′, so thatV_(N,out) presents a continuity of at least its first and secondderivates. This improves the longevity of the components and thecleanliness of the output.

Preferably, the series of controllers consists of one controller unit.As opposed to individual controllers for each converter, this allowsmanaging more efficiently and easily the energy flow, without the needof adding additional components to manage communication betweenindividual controllers.

Preferably, the power backup processor unit further comprises a DC/DCconverter connected to the first and second DC voltage busses forconnecting said first and second DC voltage busses to said DC powersource. This allows the use of a DC power source with a different,usually lower, voltage than the voltage between the two DC voltagebusses.

In a particular embodiment, the DC/DC converter is bi-directional. Thismakes it possible to use the device according to the invention as arectifier, wherein current would flow from the AC input to the DC powersource for recharging.

Advantageously, the first, second, fifth and sixth switches are powerbi-directional and the series of controllers is provided to run thefirst half-bridge circuit in reverse, so as to convert a DC voltagebetween the two DC voltage busses into an AC voltage at the first ACinput line. This allows using the device according to the invention as ameans to supply energy from the DC power input into the AC power supply.A current flow in this direction could be applied for example inphotovoltaic cell applications, wherein the energy generated by thecells is fed back to the AC power supply when not used by the criticalload.

It can further be advantageous if the third, fourth and, if provided,the seventh and eighth switches are power bi-directional, and the seriesof controllers is also provided to run the second half-bridge circuit inreverse, so as to convert an AC voltage between the first and second ACoutput lines into a DC voltage between the two DC voltage busses. Thisallows the recuperation of energy from the critical load back into theAC and/or DC power supply, which principle could be applied to elevatorsystems.

Preferably, the power backup processor according to the inventioncomprises a plurality of power backup processor units connected inparallel, each power backup processor unit comprising the first, secondand third converters as claimed in any one of the preceding claims. Thisallows to guarantee continuous operation should one of the units fail tooperate and without the presence of a single point of failure elementsuch as switch 105 a in the prior art device of FIG. 1.

The device according to particular embodiments of the invention allowsto suppress the following two negative effects occurring simultaneouslyand present in the prior art devices as illustrated in FIG. 1. First,when power is supplied through switch 105 b and cable 107, disturbancescan occur by supplying power to the critical load without filtering.Secondly, when a critical non linear load is supplied, harmonic currentsare generated on the AC power supply. This can lead to deterioration ofthe quality of the AC power supply voltage caused by harmonic currentswhich are applied to the AC power supply.

Other details and advantages will appear from the description of theannexed drawings.

FIG. 1 illustrates a typical configuration of a power backup processoraccording to the prior art.

FIG. 2 illustrates a first embodiment of the power backup processor unitaccording to the invention.

FIG. 3 illustrates part of said first embodiment of the power backupprocessor unit according to the invention.

FIG. 4 illustrates voltage conversion using pulse width modulation.

FIG. 5 illustrates an example of the output voltages of the power backupprocessor unit according to the invention.

FIG. 6 illustrates another example of the output voltages of the powerbackup processor unit according to the invention.

FIG. 7 illustrates a second embodiment of the power backup processoraccording to the invention.

FIG. 8 illustrates part of said second embodiment of the power backupprocessor according to the invention.

FIG. 9 illustrates a further embodiment of the power backup processoraccording to the invention with several units in parallel.

In a first embodiment of the invention, as illustrated in FIG. 2, apower processor unit 200 comprises a first converter 201 comprising anAC input connected to an AC power supply 205. This converter is providedto convert AC power at for example 230 V AC RMS to a DC power at forexample 350-400 V DC. The DC output of this first converter is connectedto a DC input of a second converter 202. The second converter 202 isprovided to convert de DC power to AC power of for example 230 V AC RMS,which is then fed to the critical load 204. The path formed by the firstand the second converters is used when power needs to be fed from the ACpower supply 205.

The power processor unit 200 is further provided to receive power from aDC power supply 206 such as a battery providing a DC power at forexample 48 V DC. Since the DC input of the second converter usually hasa considerably higher voltage, the power processor unit 200 will alsocomprise a third converter 203 comprising a DC input provided to beconnected to the DC power supply 206 and a DC output. The thirdconverter 203 is preferably an isolated converter (as represented by thedouble diagonal line), but could in some application be non isolated.According to the invention and as illustrated in FIG. 2, this DC outputis connected to both the DC output from the first converter 201 and theDC input from the second converter. The path formed by the third and thesecond converters is thus used when power needs to be fed from the DCpower supply 206.

A capacitor 207 is provided between the output of the first converter201 and the input of the second converter 202 and acts an energy storageunit. A micro-controller or controller 208 is connected to each of thethree converters 201, 202 and 203 to communicate commands to each of theconverters. The micro-controller is further connected to communicationoutput 209.

FIG. 3 shows in more detail the first and second converters 201 and 202of the power processor according to this first embodiment of the presentinvention. The first converter 201 comprises a first and a thirdhalf-bridge circuit 301, 303, both connected between a first and asecond DC voltage bus 311, 312. The first half-bridge circuit 301comprises a first switch 301 a connected between an AC input line 313,comprising a first inductance 313 i, and the first DC voltage bus 311,and a second switch 301 b connected between the AC input line 313 andthe second DC voltage bus 312. The third half-bridge circuit 303comprises a fifth switch 303 a, connected between a neutral line 314 andthe first DC voltage bus 311, and a sixth switch 303 b, connectedbetween the neutral line 314 and the second DC voltage bus 312. Thesecond converter 202 comprises a second and a fourth half-bridge circuit302,304, both also connected between the first and the second DC voltagebus 311, 312. The second half-bridge circuit 302 comprises a thirdswitch 302 a, connected between a first AC output line 315, comprisingan inductance 315 i, and the first DC voltage bus 311, and a fourthswitch 302 b, connected between the first AC output line 315 and thesecond DC voltage bus 312. The fourth half bridge circuit 304 comprisesa seventh switch 304 a, connected between a second AC output line 316and the first DC voltage bus 311, and an eighth switch 304 b, connectedbetween the second AC output line 316 and the second DC voltage bus 312.The DC/DC converter 203 and the capacitor 207 are then connected to thefirst and second DC voltage busses 311, 312 between the first and secondconverters 201 and 202.

To convert the cyclical, substantially sinusoidal voltage of the ACpower supply into a continuous voltage between the first and second DCvoltage busses 311, 312, and to convert said continuous voltage backinto a cyclical voltage between the first and second AC output lines315,316, the micro-controller 208 commands the switches of at least thefirst and second half-bridge circuits 301,302 using pulse widthmodulation (PWM). Pulse width modulation uses a square wave whose dutycycle is modulated resulting in the variation of the average value ofthe waveform. By switching voltage with the approximate duty cycle, theoutput will approximate a voltage at the appropriate level, as depictedin FIG. 4. The first and second inductances 313 i, 315 i and thecapacitor 207 attenuate the switching noise. The duty cycle of thesquare waveform is much shorter than that of the sine wave of either theinput or output AC voltages, typically in the order of magnitude ofmicroseconds, e.g. 20 μs vs. 20 ms for a typical 50 Hz AC voltage.

When the first AC input line 313 is connected to a single-phase AC powersupply, a variable, cyclical voltage V_(L,in) is established at thefirst AC input line 313 in the form of a substantially sinusoidal wave.This AC voltage V_(L,in) has a frequency f_(L,in), and therefore a cyclelength 1/f_(L,in), and an amplitude V_(L,inPeak). V_(L,inPeak) can be,for example 325 V, corresponding to an RMS AC voltage V_(L,inRMS) of 230V, and f_(L,in) can be, for example, 50 Hz. During the first half ofeach cycle of V_(L,in), the controllers leave the fifth switch 303 aopen, while the sixth switch 303 b is closed so as to connect the secondDC voltage bus 312 to the neutral line 314 and the first half-bridgecircuit 301 is pulse width modulated to convert the variable positivevoltage at the first AC input line 313 into a continuous voltage +V_(b)at the first DC voltage bus 311. During the second half of each cycle ofV_(L,in), the controllers close the fifth switch 303 a, while the sixthswitch 303 b is opened so as to connect the first DC voltage bus 311 tothe neutral line 314, and the first half-bridge circuit 301 is pulsewidth modulated to convert the variable negative voltage at the first ACinput line 313 into a continuous voltage −V_(b) at the second DC voltagebus 312. As a result, a substantially continuous voltage difference2V_(b) is generated between the first DC voltage bus 311 and the secondDC voltage bus 312. The first inductance 313 i filters the switchingnoise of the first and second switches 301 a, 301 b.

In the second half-bridge circuit 302, the third switch 302 a and thefourth switch 302 b, respectively connecting the first DC voltage bus311 and the second DC voltage bus 312 to the first AC output line 315,are pulse width modulated so as to generate a cyclical, close tosinusoidal voltage V_(L,out) at the first AC output line, with afrequency f_(L,out), an amplitude V_(L,outPeak) close to V_(b) and aphase displacement Δφ with respect to V_(L,in). The second inductance315 i filters the switching noise of the third and fourth switches 302a, 302 b.

In this first embodiment of the power backup processor of the invention,as illustrated in FIG. 3, the seventh switch 304 a is open during atleast part of the first half of each cycle of V_(L,out) and closedduring at least part the second half. Inversely, the eighth switch 304 bis closed during at least part of the first half and opened during atleast part of the second half. As a result, a counter-cyclical voltageV_(N,out) is generated at the second AC output line 315.

FIG. 5 illustrates a cycle of the output voltages V_(L,out), V_(N,out)and V_(load)=(V_(N,out)−V_(L,out)). During a period t₁′ of at least 95%of the first half-cycle, during which the seventh switch 304 a is openand the eighth switch 304 b closed, V_(N,out) is substantially constantand equal to −V_(b). During a period t₂′ of at least 95% of the secondhalf-cycle, during which the seventh switch 304 a is closed and theeighth switch 304 b open, is V_(N,out) substantially constant and equalto +V_(b). During the transition period t₃′ between t₁′ and t₂′, thisfourth half-bridge circuit 304 is preferably pulse-width modulated toensure a smooth transition between −V_(b) and +V_(b) and vice versa. Asimilar transition period also takes place between t₂′ and t₁′. Ideally,both the first and the second derivatives of V_(N,out) are continuous.Each one of the periods t₁′ and t₂′ can be continuous or split inseveral segments of, respectively, the first or second half-cycles. Theycan also be centred around the peaks of V_(L,out), as depicted, or, onthe contrary, advanced or delayed with respect to those peaks and havethe same or different lengths.

As V_(N,out) is countercyclical to V_(L,out), the output voltageV_(load)=(V_(N,out)−V_(L,out)) supplied to the critical load has nearlytwice the amplitude of V_(N,out), as depicted in FIG. 5. This allows acomparatively high output voltage V_(load) with a comparatively moderateDC voltage difference 2V_(b) between the first DC voltage bus 311 andthe second DC voltage bus 312 If V_(N,out) is substantially square ortrapezoidal, it is preferable to adapt the pulse width modulation of thesecond half-bridge circuit 302 so that V_(L,out) has a correctedsinusoidal form that in combination with V_(N,out) results in a closerto sinusoidal V_(load)=(V_(N,out)−V_(L,out)), as illustrated in FIG. 5.

Depending on the length of t₁′ and t₂′, the slope of V_(N,out) between−V_(b) and +V_(b) will be more or less pronounced, leading to a moresquare or trapezoidal wave shape, as can be seen by comparing FIG. 5 andFIG. 6, corresponding to an alternative where each one of t₁′ and t₂′correspond to less than 95% but at least 75% of each half-cycle. Havingsuch a substantially square or trapezoidal wave shape for V_(N,out)would result in a distorted wave shape of V_(load) if the secondhalf-bridge circuit 302 was pulse-width modulated to produce asinusoidal wave shape of V_(L,out). For this reason, the secondhalf-bridge circuit 302 is commanded so that V_(L,out) follows acorrected sine wave, as depicted in FIGS. 5 and 6, resulting in asubstantially sinusoidal V_(load). While the fourth half-bridge circuit304 could instead be pulse-width modulated during the whole cycle, so asto generate a sinusoidal V_(N,out) that would not distort V_(L,out),this would be considerably less energy-efficient than pulse-widthmodulating only the second half-bridge circuit 302 for the whole cyclewith a corrected wave shape instead. The shorter the transition periodsbetween t₁′ and t₂′, the bigger the correction of the wave shape ofV_(L,out) will have to be. On the other hand, shorter transition periodsbetween t₁′ and t₂′ have the advantage of limiting the number ofswitchovers of the fourth half-bridge circuit 304, with thecorresponding advantages in higher efficiency, lower radio interferencegeneration, etc.

It is particularly advantageous to command both the third and the fourthhalf-bridge circuits 303, 304 using this method. However, it is alsopossible to command just one of them in this manner.

FIGS. 7 and 8 illustrate a simplified second embodiment of the powerbackup processor of the invention. In this second embodiment, the secondhalf-bridge circuit 302 is synchronised with the first half-bridgecircuit 301, so that V_(L,out) has substantially, or even precisely, thesame frequency and phase as V_(L,in). Since the third half-bridgecircuit 303 is controlled so that the neutral line 314 is connected tothe second DC voltage bus 312 during the first half of the cycle ofV_(L,in) and therefore, in this embodiment, also of V_(L,out), and tothe first DC voltage bus 311 during the second half of the same cycle,this allows to dispense with the fourth half-bridge circuit 304 of thefirst embodiment and to connect the second AC output line 316 directlyto the neutral line 314 instead. However, in this embodiment a thirdimpedance 314 i has to be interposed between the neutral line 314 andthe third half-bridge circuit 303 to filter the switching noise of thefifth and sixth switches 303 a, 303 b.

As shown in FIG. 7, this forms, with the capacitor 207, one AC/DC/ACconverter 701.

In this second embodiment, the third half-bridge circuit 303 is alsocontrolled in the way depicted by FIG. 5 or 6, wherein during a periodt₁ of at least 95%, respectively at least 75% of the first half-cycle,during which the fifth switch 303 a is open and the sixth switch 303 bclosed, V_(N,out) is substantially constant and equal to −V_(b). Duringa period t₂ of at least 95%, respectively at least 75% of the secondhalf-cycle, during which the fifth switch 303 a is closed and the sixthswitch 303 b open, V_(N,out) is substantially constant and equal to+V_(b). During the transition period t₃ between t₁ and t₂, this thirdhalf-bridge circuit 304 is preferably pulse-width modulated to ensure asmooth transition between −V_(b) and +V_(b) and vice versa. Ideally,both the first and the second derivatives of V_(N,out) are continuous.Each one of the periods t₁ and t₂ can be continuous or split in severalsegments of, respectively, the first or second half-cycles. They canalso be centred around the peaks of V_(L,out), as depicted, or, on thecontrary, advanced or delayed with respect to those peaks and have thesame or different lengths.

As shown in FIGS. 3 and 8, protection means 305 are preferably providedin one of the first or second DC voltage busses 311, 312, upstream ofthe connection 210 or 211, respectively, to the DC power source 206 andof the capacitor 207. The protection means 305 consist in particular ofa fuse and allow to use the DC power source 206 when the firsthalf-bridge circuit 301 breaks and could possibly short-circuit the DCvoltage at the capacitor 207.

The DC/DC converter 203 in both the first and the second embodiments canbe bi-directional, thus allowing to direct power from the DC voltagebusses 311,312 back to the DC power source 206, for instance forrecharging a battery being used as the DC power source 206.

The first and second converters 201,202 of the first embodiment and theAC/DC/AC converter 701 of the second embodiment can also be powerbi-directional, allowing the recuperation of energy from the criticalload 204 and/or the DC power source 206. For this purpose, the switchesof the half-bridge circuits forming these first and second converterswill have to be power bi-directional themselves, and the series ofcontrollers 208 will have to be provided to run these converters 201and/or 202 or 203 also in reverse. The switches of the half-bridgecircuits can in particular be IGBT switches with antiparallel diodes.

FIG. 9 shows an embodiment from the processor according to the presentinvention comprising a plurality of processor units 200 a, 200 b, . . .200 i connected in parallel. Each of the processor units 200 a, 200 b, .. . 200 i has a configuration as shown in FIG. 2 or 7. A communicationbus 300 connects the communication outputs 209 of the processor units.Hence the micro-controllers of the processor units are interconnected toallow synchronisation of the first AC outputs and to organise loadsharing between the units. The principles of synchronisation and loadsharing between units are known to the person skilled in the art andthus do not need to be described in detail. Preferably, as illustratedin FIG. 9, the communication bus comprises a double connection betweenthe units to provided redundancy in case one of the connections wouldfail. Instead of a cable connection, wireless connection using IR or HFtechnology, in particular Bluetooth technology, is also conceivable. Itis also conceivable to provide one connection with cables and the otherconnection as a wireless connection.

As will be appreciated, modifications are conceivable without fallingoutside the scope of the appended claims. For example, it isconceivable, instead of providing one micro-controller for eachprocessor unit as illustrated in FIG. 2, one micro-controller for eachconverter 201, 202 and 203 with communication means between each of thecontrollers. Having only one controller per processor unit is howeveradvantageous, since it does not require means to ensure propercommunication between the controllers of a same processor unit. As canbe seen in the drawings, it is important that the three convertersconverge in one point: a DC bus bar at the capacitor 207.

1. A power backup processor unit comprising: a) a first AC input line comprising a first inductance and provided to be connected to an AC power supply, with a cyclical voltage V_(L,in); b) a first AC output line comprising a second inductance and provided to be connected to a critical load; c) a second AC output line provided to be connected to the critical load; d) a neutral line provided to be connected to a neutral port; e) first and second DC voltage busses provided to be connected to a DC power supply; f) a capacitor interposed between the first and second DC voltage busses; g) a first half-bridge circuit comprising a first switch and a second switch, wherein the first switch is connected between the first AC input line and the first DC voltage bus and the second switch is connected between the first AC input line and the second DC voltage bus; h) a second half-bridge circuit comprising a third switch and a fourth switch, wherein the third switch is connected between the first AC output line and the first DC voltage bus and the fourth switch is connected between the first AC output line and the second DC voltage bus; i) a third half-bridge circuit comprising a fifth switch and a sixth switch, wherein the fifth switch is connected between the neutral line and the first DC voltage bus and the sixth switch is connected between the neutral line and the second DC voltage bus; j) a series of controllers provided to command the switches, wherein said series of controllers is provided to command at least the first to fourth switches using pulse width modulation to convert V_(L,in) into two different, substantially continuous voltages +V_(b),−V_(b) at the first and the second DC voltage busses and said two different, substantially continuous voltages +V_(b),−V_(b) into a cyclical voltage V_(L,out) at the first AC output line; and characterised in that it also comprises k) a fourth half-bridge circuit comprising a seventh switch and an eighth switch, wherein the seventh switch is connected between the first DC voltage bus and the second AC output line and the eighth switch is connected between the second DC voltage bus and the second AC output line; and in that said series of controllers is also provided to open the fifth switch and close the sixth switch for a period t₁ of at least 50%, and preferably at least 95%, of a first half of a cycle of V_(L,in) and close the fifth switch and open the sixth switch for a period t₂ of at least 50%, and preferably at least 95%, of a second half of said cycle of V_(L,in).
 2. A power backup processor unit according to claim 1, characterized in that said series of controllers is also provided to open the seventh switch and close the eighth switch for a period t₁′ of at least 50%, and preferably at least 95%, of a first half of a cycle of V_(L,out) and close the seventh switch and open the eighth switch for a period t₂′ of at least 50%, and preferably at least 95% of a second half of said cycle of V_(L,out).
 3. A power backup processor unit according to claim 2, characterized in that said series of controllers is also provided to pulse-width modulate said fourth half-bridge circuit during a transition period t₃′ between t₁′ and t₂′.
 4. A power backup processor unit according to claim 1, characterised in that the third, fourth, seventh and eighth switches are power bi-directional, and the series of controllers is provided to run the second half-bridge circuit in reverse, so as to convert an AC voltage between the first and second AC output lines into a DC voltage between the two DC voltage busses.
 5. A power backup processor unit comprising: a first AC input line comprising a first inductance and provided to be connected to an AC power supply with a cyclical voltage V_(L,in); b) a first AC output line comprising a second inductance and provided to be connected to a critical load; c) a second AC output line provided to be connected to the critical load; d) a neutral line provided to be connected to a neutral port; e) first and second DC voltage busses provided to be connected to a DC power supply; f) a capacitor interposed between the first and second DC voltage busses; g) a first half-bridge circuit comprising a first switch and a second switch, wherein the first switch is connected between the first AC input line and the first DC voltage bus and the second switch is connected between the first AC input line and the second DC voltage bus; h) a second half-bridge circuit comprising a third switch and a fourth switch, wherein the third switch is connected between the first AC output line and the first DC voltage bus and the fourth switch is connected between the first AC output line and the second DC voltage bus; i) a third half-bridge circuit comprising a fifth switch and a sixth switch, wherein the fifth switch is connected between the neutral line and the first DC voltage bus and the sixth switch is connected between the neutral line and the second DC voltage bus; j) a series of controllers provided to command the switches, wherein said series of controllers is provided to command at least the first to fourth switches using pulse width modulation to convert V_(L,in) into two different, substantially continuous voltages +V_(b),−V_(b) at the first and the second DC voltage busses and said two different, substantially continuous voltages +V_(b),−V_(b) into a cyclical voltage V_(L,out) at the first AC output line; and characterised in that it also comprises k) a third inductance connected between said neutral line and said fifth and sixth switches; in that said second AC output line is directly connected to said neutral line; in that said series of controllers are provided to synchronize said first and second half-bridge circuits so that V_(L,in) and V_(L,out) have substantially the same phase and frequency, and in that said series of controllers is also provided to open the fifth switch and close the sixth switch for a period t₁ of at least 50%, and preferably at least 95%, of a first half of a cycle of V_(L,in) and close the fifth switch and open the sixth switch for a period t₂ of at least 50%, and preferably at least 95%, of a second half of the cycle of V_(L,in).
 6. A power backup processor unit according to claim 5, characterized in that said series of controllers is also provided to pulse-width modulate said fifth switch and said sixth switch during a transition period t₃ between t₁ and t₂.
 7. A power backup processor unit according to claim 5, wherein the third and fourth switches are bi-directional, and the series of controllers is also provided to run the second half-bridge circuit in reverse, so as to convert an AC voltage between the first and second AC output lines into a DC voltage between the two DC voltage busses.
 8. A power backup processor unit according to claim 1, wherein the series of controllers consists of one controller unit.
 9. A power backup processor unit according to claim 1, further comprising a DC/DC converter connected to the first and second DC voltage busses for connecting said first and second DC voltage busses to said DC power source.
 10. A power backup processor unit according to claim 9, wherein the DC/DC converter is bi-directional.
 11. A power backup processor unit according to claim 1, wherein the first, second, fifth and sixth switches are bi-directional and the series of controllers is provided to run the first half-bridge circuit in reverse, so as to convert a DC voltage between the two DC voltage busses into an AC voltage at the first AC input line.
 12. A power backup processor system comprising a plurality of power backup processor units as claimed in claim 1 and connected in parallel.
 13. The power backup processor system according to claim 12, further comprising communication means, in particular at least one inter unit communication bus, connected to the controllers of each power backup processor unit.
 14. A method of controlling a power backup processor unit, said power backup processor unit comprising: a) a first AC input line comprising a first inductance and provided to be connected to an AC power supply with a cyclical voltage V_(L,in); b) a first AC output line comprising a second inductance and provided to be connected to a critical load; c) a second AC output line provided to be connected to the critical load; d) a neutral line provided to be connected to a neutral port e) first and second DC voltage busses provided to be connected to a DC power supply; f) a capacitor interposed between the first and second DC voltage busses; g) a first half-bridge circuit comprising a first switch and a second switch, wherein the first switch is connected between the first AC input line and the first DC voltage bus and the second switch is connected between the first AC input line and the second DC voltage bus; h) a second half-bridge circuit comprising a third switch and a fourth switch, wherein the third switch is connected between the first AC output line and the first DC voltage bus and the fourth switch is connected between the first AC output line and the second DC voltage bus; i) a third half-bridge circuit comprising a fifth switch and a sixth switch, wherein the fifth switch is connected between the neutral line and the first DC voltage bus and the sixth switch is connected between the neutral line and the second DC voltage bus; and wherein said method comprises the steps of: j) pulse width modulating said first half-bridge circuit to convert V_(L,in) into two different, substantially continuous voltages +V_(b),−V_(b) at the first and the second DC voltage busses and said second half-bridge circuit to convert said two different, substantially continuous voltages +V_(b),−V_(b) into a cyclical voltage V_(L,out) at the first AC output line; and is characterised in that it also comprises the steps of k) opening the fifth switch and closing the sixth switch for a period t₁ of at least 50%, and preferably at least 95%, of a first half of a cycle of V_(L,in); and l) closing the fifth switch and opening the sixth switch for a period t₂ of at least 50%, and preferably at least 95%, of a second half of the cycle of V_(L,in).
 15. A method according to claim 14 and characterized in that it also comprises the step of pulse-width modulating said fifth switch and said sixth switch during a transition period t₃ between t₁ and t₂.
 16. A method according to claim 14 and characterized in that the first and second half-bridge circuits are synchronised so that V_(L,in) and V_(L,out) have substantially the same phase and frequency.
 17. A method according to claim 14, wherein said second AC output line is directly connected to the neutral line and said power backup processor unit also comprises a third inductance connected between said neutral line and said fifth and sixth switches.
 18. A method according to claim 14 and characterized in that said power backup processor unit further comprises: a) a fourth half-bridge circuit comprising a seventh switch and an eighth switch, wherein the seventh switch is connected between the first DC voltage bus and the second AC output line and the eighth switch is connected between the second DC voltage bus and the second AC output line; and in that the method also comprises the steps of: b) opening the seventh switch and closing the eighth switch for a period t₁′ of at least 50%, and preferably at least 95%, of a first half of a cycle of V_(L,out); and c) closing the seventh switch and closing the eighth switch for a period t₂′ of at least 50%, and preferably at least 95%, of a second half of the cycle of V_(L,out).
 19. A method according to claim 14 and characterized in that it also comprises the step of pulse-width modulating said seventh switch and said fourth switch during a transition period t₃′ between t₁′ and t₂′. 